This invention relates to vehicle service apparatus and more particularly to vehicle alignment apparatus utilizing wheel offset and body center line determinations.
It is well known that any vehicle has three lines with respect to which various alignment angles may be measured, namely, the body center line (the longitudinal axis of the vehicle frame), the geometric center line (measured with respect to the wheels and defined as the line connecting two mid-points at the front and rear axles), and the thrust line (a line perpendicular to the average axes of the wheels of the drive axle). The ideal line with which to work is the body center line. Unfortunately it is the least accessible since it depends upon the orientation of the body or frame of the vehicle, while conventional alignment apparatus instead measure the orientation of the wheels of the vehicle. Heretofore, most alignment equipment has simply assumed that the geometric center line and the body center line were coincident because of the difficulty in determining the true body center line.
At least one aligner designed for multi-axle trucks does provide for a determination of the body center line, but that aligner could be improved. Moreover, the procedure used therein is not particularly suitable for smaller vehicles which do not generally have an exposed frame available for reference by the technician/user. In this prior art aligner, strings extend from the front to the rear axles and provide reference lines. Automobile aligners typically have not used strings for many years. The user of this prior art aligner must make a measurement from the string to the truck frame, being careful to make the measurement in a horizontal plane and at a ninety degree angle from the frame. Since the strings are movable and the user must estimate whether the measurement is being taken in a horizontal plane and whether the measurement is being taken at ninety degrees to the frame, this prior art procedure is subject to error and could be improved.
Since the body center line is not readily available to the user, related angles such as a vehicle dog track angle and the vehicle setback angles must be estimated rather than measured.